The invention relates to a bath deposition solution for the chemical bath deposition of a metal sulfide layer, to a process for the production of such a bath deposition solution and to a process for producing a metal sulfide layer on a substrate using such a bath deposition solution. The invention is particularly suitable for the chemical bath deposition (CBD) of a zinc sulfide layer (known as CBD zinc sulfide layer) as buffer layer on an absorber layer in the manufacture of photovoltaic thin film components. For the present purposes, such CBD zinc sulfide layers are layers which can contain not only zinc and sulfur but also, due to the method of production, oxygen in a proportion dependent on the preparation conditions, and the layers are then also referred to formally as Zn(S,O) layers or ZnSi1-xOx layers where 0≦x<1 or Zn(S,O,OH) layers or ZnS(O,OH) layers, in the technical literature.
Especially for said ZnS buffer layer deposition in solar cell applications, various procedures have already been proposed. Thus, for example, it is known from the laid-open publications WO 2006/018013 A1 and DE 10 2006 039 331 A1 that a bath deposition solution prepared in a specific way from zinc sulfate, thiourea and ammonia dissolved in distilled water can be used for this purpose, where the temperature of the deposition solution is maintained at from 70° C. to 90° C. during the deposition process or is ramped up to such a temperature value. On the basis of experience, production of a ZnS buffer layer having a layer thickness in the order of 25 nm as is typically required requires a deposition time of at least about 15 minutes when using this deposition process.
ZnS buffer layers have recently been found to be very useful as an alternative to cadmium sulfide (CdS) buffer layers. Here, ZnS buffer layers are known to be less problematical than CdS buffer layers from an environmental point of view and transparent buffer layers can be produced therefrom. Compared to CdS, ZnS has a higher band gap and absorbs very little in the wavelength range from 300 nm to 500 nm. As a result, more light reaches the photovoltaically active absorber layer, which leads to a higher current density and a potentially higher efficiency. Furthermore, there has recently been increasing demand for the commercial manufacture of large-area photovoltaic modules, e.g. of the CIS or CIGS type. There is therefore great interest in a rapid and inexpensive process by means of which ZnS buffer layers can be deposited in the required layer quality over a large area of an appropriate substrate, i.e. a corresponding solar cell absorber layer. CBD zinc sulfide layers are of particular interest for this purpose, especially also when they contain oxygen as a result of the method of manufacture, as indicated above. The conduction band offset relative to the adjacent CIS absorber layer material can in this way be reduced from about 1.6 eV to about 1.0 eV or less. Depending on the oxygen content and any hydrogen or hydroxide content, the CBD zinc sulfide layers can be binary compounds composed of the elements Zn and S, ternary compounds composed of the elements Zn, O and S or the constituents Zn, S and OH, quaternary compounds composed of the constituents Zn, S, O and OH or the constituents Zn, S, O and H or oxygen- and/or hydrogen-doped ZnS layers, with continuous transitions between these types of compound naturally also being possible depending on the proportion or doping concentration of the constituent concerned.
The journal article R. Sahraei et al., Compositional, structural, and optical study of non-crystalline ZnS thin films prepared by a new chemical bath deposition route, J. of Alloys and Compounds 466 (2008), page 488, discloses a ZnS buffer layer deposition process in which a weakly acidic solution having a pH of about 5 and containing zinc chloride (ZnCl2), nitrilotriacetic acid (NTA) thioacetamide (TAA) and sodium hydroxide (NaOH) to adjust the pH is used as bath deposition solution. Deposition is carried out for up to about 6 hours at a temperature of about 70° C. in order to achieve a layer thickness of about 80 nm, with this deposition process being repeated if required in order to achieve greater layer thicknesses.
The journal article A. Goudarzi et al., Ammonia-free chemical bath deposition of nanocrystalline ZnS thin film buffer layer for solar cells, Thin Solid Films, 516 (2008), page 4953, discloses a ZnS buffer layer deposition process in which an ammonia-free, weakly acidic bath deposition solution having a pH of about 6.0 and containing zinc acetate, TAA, NaOH for adjusting the pH and a sodium salt of ethylenediaminetetraacetic acid (Na2EDTA) dissolved in distilled water is used. Layer thicknesses of from about 20 nm to 140 nm are achieved for the ZnS layer deposited in deposition times of from about 30 minutes to seven hours using this bath deposition solution.
In the journal article S. Nagalingam et al., The Effect of EDTA on the Deposition of ZnS Thin Film, Z. Phys. Chem. 222 (2008), page 1703, electrodeposition of a ZnS buffer layer in a distinctly acidic deposition solution having a pH of not more than 4 and preferably about 1.26, where the deposition solution contains ZnCl2, Na2S2O3, NaEDTA and hydrochloric acid (HCl) to adjust the pH dissolved in distilled water, is proposed as an alternative to electroless chemical bath deposition.
Laid-open publication US 2007/0020400 A1 discloses a process for the continuous deposition of thin layers, for example ZnS and CdS layers, using a micromixer and a microchannel applicator. In this micromixer, two preferably liquid reactants are mixed in order to provide the deposition material desired for the deposition, for example a solution of cadmium chloride, ammonium chloride and ammonium hydroxide as first reactant and aqueous urea as second reactant for the deposition of a CdS layer. The deposition Material provided, e.g. in the form of appropriate CdS particles for the deposition of a CdS layer, is then directed in the form of a jet onto the surface to be coated by means of the microchannel applicator.
The technical problem addressed by the invention is to provide a bath deposition solution, an accompanying process for producing the solution and also a metal sulfide layer production process using this, which allows the electroless, chemical bath deposition of a metal sulfide layer of good quality, as is required, for example, for ZnS buffer layers in solar cell applications, with a short deposition time and which are also suitable, in particular, for large-area deposition as is required, for example, for ZnS buffer layers in the manufacture of large-area photovoltaic modules.
The invention solves this problem by providing a bath deposition solution for the chemical bath deposition of a metal sulfide layer, wherein it contains: a metal salt, an organosulfide, a chelating agent which with metal ions of the metal salt forms a chelate complex and ammonium hydroxide. The invention further solves this problem by providing a bath deposition solution production process for producing a bath deposition solution for chemical bath deposition of a metal sulfide layer, wherein a metal salt, an organosulfide, a chelating agent which with metal ions of the metal salt forms a chelate complex and ammonium hydroxide are mixed in distilled water. The invention still further solves this problem by providing a metal sulfide layer production process for producing a metal sulfide layer on a substrate, which comprises the following steps: provision of the inventive bath deposition solution, and chemical bath deposition of the metal sulfide layer on the substrate by bringing the substrate into contact with the bath deposition solution.
The bath deposition solution for the chemical bath deposition of a metal sulfide layer characteristically comprises a salt of the metal required for the metal sulfide layer and also an organosulfide as sulfur source for the metal sulfide layer, a chelating agent which forms a chelated complex with metal ions of the metal salt and ammonium hydroxide. One or more further constituents can optionally be present in a low concentration.
It has been found that the presence of the organosulfide as supplier of sulfur and also both the chelating agent and ammonium hydroxide makes it possible to achieve very advantageous, fast deposition of a metal sulfide layer of good quality even on relatively large areas, for which the interaction of the chelating agent, which due to its strong shielding effect makes it difficult for the metal ions to access the sulfur ions liberated from the organosulfide, and the ammonium hydroxide is considered to be responsible, with the ammonium hydroxide not only acting as agent to adjust the pH but also functioning as weak metal ion shielding component. The inventors have found that this composition of the bath deposition solution enables, for example, ZnS buffer layers having a thickness of about 25 nm and a good layer quality to be deposited in a deposition time of less than about 10 minutes, in particular only about 4 minutes.
While a Zn salt is used as metal salt for the deposition of ZnS layers such as said buffer layers for solar cell applications, it is possible, according to the invention, for other metal sulfide layers, e.g. of In or combinations of the metals Zn and In, to be chemically bath deposited in the same way using corresponding metal salts.
In an advantageous embodiment, thioacetamide (TAA) is used as organosulfide. It has been found that it is in this way possible to achieve more rapid liberation of sulfur, for example compared to thiourea, which can be explained by a higher hydrolysis sensitivity of TAA. An excessively rapid metal sulfide formation reaction associated with undesirable colloidal deposits of the metal sulfide or poor covering/morphology characteristics is prevented by controlled slowing of the attachment of sulfur to the metal ion due to the shielding effect of the chelating agent and of the ammonium hydroxide or ammonia.
In an advantageous embodiment of the invention, nitrilotriacetic acid (NTA) or iminodiacetic acid (IDA) or an appropriate salt of these acids, e.g. a sodium salt, ammonium salt etc. is used as chelating agent. It has been found that the use of such a chelating agent in combination with a suitable organosulfide, e.g. TAA, makes it possible to achieve good matching of the rate of liberation of sulfur by the organosulfide and the attachment of sulfur to the metal ion which has been slowed by the chelating agent in such a way that the metal sulfide layer can be deposited with comparatively very good quality and a high deposition rate of, for example, about 6 nm/min.
In an embodiment of the invention, the metal salt is present in a concentration of from about 1 mM to about 50 mM and/or the organosulfide is present in a concentration of from about 1 mM to about 150 mM and/or the chelating agent is present in a concentration of from about 0.01 M to about 1.0 M and/or the ammonium hydroxide is present in a concentration of from about 0.01 M to about 3.0 M in the bath deposition solution. These concentration ranges have been found to be particularly useful for achieving metal sulfide layers of good quality at a satisfactorily high deposition rate combined with a comparatively low usage of materials.
In a further embodiment of the invention, a pH in the basic range through to the neutral range is set for the bath deposition solution. It has been found that this enables advantageous chemical bath deposition of the metal sulfide layer to be achieved.
The bath deposition solution of the invention can, according to the invention, be produced easily by simple mixing of the participating constituents in distilled water.
When the bath deposition solution of the invention is used, it is possible, as mentioned, for metal sulfide layers to be chemically bath deposited on a substrate with comparatively good layer quality and a high deposition rate. Thus, for example, a ZnS buffer layer can be deposited on a photovoltaic absorber layer substrate by means of this process of the invention, with the layer quality required for such a buffer layer and a layer thickness of about 25 nm being able to be achieved at very short deposition times of not more than about 10 minutes and preferably not more than about 4 minutes or less.
In an embodiment of this metal sulfide layer production process according to the invention, the bath solution is maintained at a temperature of from about 40° C. to about 90° C. during the deposition process.
In a further embodiment of the invention, the contacting of the substrate with the bath deposition solution is effected by dipping the substrate into the bath deposition solution or equivalent contacting of the area of the substrate surface to be coated with the bath deposition solution by means of an appropriate wetting or spraying technique.